In magnetic recording disks, increases in areal density have been accomplished by scaling down the area of a data bit by reducing the grain size of the magnetic particles in the recording medium. However, reducing the grain size of the magnetic particles also reduces the magnetic stability of the particles. At the superparamagnetic limit, the grain size of the magnetic particles become so small that spontaneous magnetic reversal under slight thermal agitation may occur. To counter this superparamagnetic limit, higher coercivity magnetic materials may be used in the recording medium. With the use of higher coercivity magnetic materials, the application of a strong magnetic field may be required during write operations to magnetize the recording medium. However, conventional magnetic write heads may not be able to provide the required magnetic field intensity due to physical limitations of the write heads.
In energy-assisted magnetic recording (EMR), the recording medium is locally heated to decrease the coercivity of the magnetic material during write operations. The local area is then rapidly cooled to retain the written information. This allows for conventional magnetic write heads to be used with high coercivity magnetic materials. The heating of a local area may be accomplished by, for example, a heat or thermal source such as a laser. As such, one type of energy-assisted magnetic recording is heat assisted magnetic recording (HAMR). HAMR may also sometimes be referred to as thermally assisted magnetic recording (TAMR) or optically assisted magnetic recording (OAMR).
FIG. 1A illustrates a cross sectional view of a write head 120, a laser source 130, and a continuous PMR disk 100 inside a conventional EMR drive 150. The PMR disk 100 may include a substrate 101, a soft underlayer 102, nucleation layers 103, and a magnetic recording layer 104. The write head 120 may include a writer yoke 121 and writer coils 122. The laser source 130 may include a waveguide 131 to generate a laser beam 110. A near-field transducer (NFT) 132 may be coupled to the waveguide 131 to focus the laser beam 110 into a laser spot 111 on a continuous PMR disk 100 during the write operation. The heat from the laser spot 111 may form a thermal spot 112, which represents the area that is locally heated during the write operation. Consequently, the minimum recording track width is determined by the size of the thermal spot 112, because if the recording tracks are made smaller than the thermal spot 112, then adjacent tracks may be inadvertently written due to thermal spreading of the thermal spot 112 into adjacent tracks.
For example, to achieve an areal density of 1 terabit per square inch, a thermal spot size of about 43 nanometers (nm) may be required. In conventional energy-assisted magnetic recording, the thermal spot 112 size may be reduced by reducing the laser spot 112 size through a reduction in the NFT 132 dimensions. To achieve the thermal spot 112 size of 43 nm with a continuous PMR disk 100, the NFT 132 dimension requirement is about 20 nm. As the areal density increases further, the minimum distance requirement for the NFT 132 approaches zero. This creates increasing difficulty in the lithographic process for manufacturing the NFT 132.
FIG. 1B illustrates the thermal spot 112 size on a continuous PMR disk 100 in a conventional EMR drive 150 during a write operation. Inside the thermal spot 112, darker shading indicates hotter temperatures. In this particular example, the laser source 130 produces a laser spot 111 having a full-width half maximum (FWHM) of 40 nm. The resulting FWHM of the thermal spot 112 is about 57.5 nm, or close to 20 nm wider than the laser spot 111. This thermal spreading in the conventional EMR drive 150 may be due to the poor thermal conductivity of the intermediate layers 103 of the PMR disk 100.
To reduce the thermal spreading to adjacent tracks in EMR, different disk structures have been developed and discussed, for example, in U.S. Patent Application, 2006/0210838 A1 to Kamimura et al. (hereinafter “Kamimura”) and U.S. Patent Application 2007/0279791 A1 to Mallary (hereinafter “Mallary”). In Kamimura, heat conductive grooves are sunken into the substrate from the magnetic recording layer to assist with dissipating heat from the disk surface into the substrate. However, the depth of the grooves being all the way down into the substrate may pose challenges in fabrication and in uniform deposition of the heat conductive material into the grooves.
In Mallary, air gaps are formed in grooves that extend down to the soft underlayer. A thermal insulator layer is also inserted above the soft underlayer. However, the use of air gaps above a thermal insulator layer may slow the propagation of heat out of the magnetic recording layer. This may increase the cooling time for a previously written area. As a result, the previously written area may be inadvertently erased if the same area is subsequently exposed to a magnetic field before sufficient cooling has occurred.